1. Field of the Invention
This invention relates to a machine for dressing tools used in the grinding and polishing of optical and ophthalmic lenses having a toric surface configuration. More particularly, it relates to a novel machine for generating or restoring the accurate toric form of fixed-super-abrasive tools employed in the production of toric lenses, such as is described in the copending patent application of Carle W. Highberg, U.S. Pat. Ser. No. 569,303, filed Apr. 17, 1975.
While the present invention is described herein with reference to a particular embodiment, it should be understood that the invention is not limited thereto. The elements of the present invention may be employed in a variety of structural forms, as those skilled in the art will recognize in the light of the present disclosure.
As used herein, the term "super-abrasive" refers to abrasive media suitable for grinding and polishing conventional ophthalmic and optical lens glass and having a hardness on the Knoop scale in excess of about 3,000 kg/mm.sup.2, preferably substantially in excess thereof. A comparison of Knoop and Mohs hardness values for conversion purposes is available in standard handbooks. Commercially known super-abrasives include natural and synthetic industrial diamonds and cubic boron nitride. These contrast with "conventional abrasives" of limited hardness, i.e., a hardness less than about 3,000 kg/mm.sup.2. Commercially available conventional abrasives include, for example, garnet, silicon carbide, emery, alumina, zirconium oxide, cerium oxide, or the like.
The term "fixed", as used herein in connection with either super-abrasives or conventional abrasives, refers to the disposition of the abrasive particles in the form of a solid or bonded tool in contrast to being disposed in a liquid slurry or similar fluid medium. As will be apparent from the description herein, the machine of the present invention is specifically designed to use a fast-wearing fixed conventional abrasive in the form of a grinding wheel for dressing the toric surface of a fixed-super-abrasive tool.
2. Description of the Prior Art
In the aforementioned copending U.S. Pat. application Ser. No. 569,303, a novel fixed-super-abrasive tool for the grinding of optical and ophthalmic lenses and the like, particularly toric-configured lenses, and the method of making the same have been described in detail. In brief, it comprises a rigid backing member having a malleable wafer releasably secured thereto, the malleable wafer comprising super-abrasive particles, e.g., diamonds, in a powdered metal matrix, e.g., a powdered tin-copper alloy matrix.
Upon the initial manufacture thereof, or after substantial use resulting in detectable wear, the working surfaces of these fixed-super-abrasive tools may require a dressing operation to generate or restore the toric curvature within the required precise tolerances. The dressing of these toric-configured, fixed-super-abrasive tools presents unique problems for which prior-art dressing or shaping machines have not been designed and which are not capable of coping therewith.
There are, for example, conventional lap cutters and shaping machines for generating toric curvatures on conventional grinding and polishing tools, such as cast iron grinding tools used in connection with liquid slurries containing conventional abrasives. Such toric shaping machines are available commercially from suppliers of optical processing equipment, e.e., Shuron-Continental Division of Textron, Inc., Coburn Optical Industries, Inc., and others.
In these conventional shapers, the cutter, e.g., a single point tungsten caribide tip, oscillates in a vertical plane at a first predetermined radius, the motion being essentially a vertical arc. Simultaneously, the workpiece, e.g., the cast iron tool on which the toric surface is being generated, is moved across the path of the vertically-oscillating cutter in a horizontal plane at a second predetermined radius, the motion being essentially a horizontal arc at a second predetermined radius. The combined dressing action about two axes at right angles to each other generates the desired toric surface on the workpiece.
Manifestly, it is critically important that the radii at which the combined dressing action occurs remain constant. In a conventional shaping operation, the locus of the cutting action is set by the position of the tip of the tungsten carbide tool. This presents no problem when shaping a conventional cast iron workpiece because the tungsten carbide shaping tip is so hard relative to the cast iron workpiece and therefore no significant wear of the tip occurs. Thus no significant change in the radii occurs.
When the workpiece contains fixed-super-abrasives, however, the much greater hardness of the workpiece wears away the cutter at a fast rate. This reduces the radii at which the cutting action occurs. This in turn changes the curvature in both directions and results in undesired and unacceptable distortion of the toric surface being dressed.
Even if the tungsten carbide tip were replaced with a single point fixed-super-abrasive tool, the tip itself would still wear away at a rate comparable to that of the super-abrasive tool being dressed. This would also be unacceptable. Moreover, for most purposes, the single point super-abrasive tip would be worn away at a much faster rate because the contacting area of the single point tip would be a much smaller area than the surface area being dressed, thus concentrating all of the wear in a small area.